The present invention relates to a power assisted steering apparatus that is applicable to an automotive vehicular steering system.
Generally, in order to facilitate a steering operation through a steering wheel with less force, various types of power assisted steering apparatuses in which supply and exhaust of a working oil (hydraulic) into or from a (hydraulic) actuator installed in the vehicular steering system are carried out to achieve a steering assisted power have been used.
A Japanese Patent Application First Publication No. Showa 61-85272 exemplifies one of the previously proposed power assisted steering apparatuses provided with an actuator constituted by a regulating valve and a power cylinder, a hydraulic circuit interposed between an oil pump driven by means of an electric motor and the actuator, an accumulator installed in the hydraulic circuit, a pressure sensor arranged for detecting a hydraulic pressure in the hydraulic circuit, a controller arranged for receiving a detection signal from the pressure sensor and controlling a drive or stop of the motor according to the received detection signal.
In the disclosed vehicular power assisted steering apparatus, when the pressure sensor detects a reduction in the hydraulic pressure in the hydraulic circuit, the working oil is accumulated into the accumulator so that, during an abrupt steering operation, an inconvenience for a discharge oil quantity to become insufficient can be eliminated because of its rise characteristic of the hydraulic pump in a start operation.
However, in the previously proposed power assisted steering apparatus disclosed in the above-identified Japanese Patent Application First Publication, the hydraulic pump for the working oil is driven and rotated by means of the electric motor so that, in the automotive vehicle in which an internal combustion engine is mounted, a driving energy of the engine is not only reduced according to a working efficiency of the motor but also is reduced according to a generation efficiency of the motor but also is reduced according to a generation efficiency of an engine alternator. Consequently, an energy loss may become large. In addition, since the electrical power consumption of the motor is generally large so that the energy consumption, i.e., the fuel consumption of the engine may accordingly be worsened.
In addition, a remaining pressure in a hydraulic circuit (working oil pressure passage) held by means of an accumulator is maintained at a predetermined pressure determined according to a capacity of the accumulator. In other words, in a state wherein the variable capacity pump discharges the working oil pressure, the accumulator always accumulates the working oil until the accumulated working oil has reached to a maximum capacity that the accumulator has (the maximum capacity depends on a maximum pressure and maximum volume of the accumulator).
Anyway, an energy that requires in the actuator during a steering operation is determined according to a predetermined working oil pressure based on a regulation of a regulating valve of the actuator and a stroke variable of the power cylinder that the actuator has. Hence, suppose that the power cylinder is operated according to the energy accumulated in the accumulator. In a case where the power cylinder strokes at a relatively short distance with a working oil pressure equal to a maximum pressure in the accumulator, the energy accumulated in the accumulator is efficiently utilized. On the contrary, in a case where only a small steering assistance force is required and the power cylinder strokes at a relatively long distance with the working oil pressure smaller than the maximum pressure of the accumulator, the energy accumulated in the accumulator is not effectively utilized.
In other words, although the accumulator is always driven to accumulate the working oil pressure up to the maximum volume thereof, the power cylinder is not always operated with the maximum working pressure. When the power cylinder is required to provide a relatively small steering assistance force, the working pressure of the power cylinder of the actuator may be small. In this case, the accumulation energy of the accumulator may be small. That is to say, when the power cylinder of the accumulator requires only a small working pressure, the variable capacity pump consumes wastefully the energy since an extremely larger energy than the working pressure is accumulated in the accumulator.
It is noted that, in the previously proposed power assisted steering apparatus, in order to avoid a wasteful power consumption if the maximum pressure is accumulated in the accumulator, a sufficient steering assistance force required during a start of the steering operation through a steering wheel of the vehicle cannot be obtained if grounding resistances of road wheels of the vehicle are large. Hence, it is logically possible for the maximum pressure that the accumulator has to be reduced without limitation.